1. Field of the Invention
The present invention relates to a mechanism for automatically returning a movable swash plate to the neutral position in an axle driving apparatus which drives the axles thereof by a hydrostatic transmission (hereinafter referred to as an xe2x80x9cHSTxe2x80x9d) for steplessly changing the rotational speed of an engine, wherein resistance is given to the movable swash plate and pressure generated in a closed fluid circuit of the HST escapes therefrom when the movable swash plate returns to the neutral position, so as to prevent the vehicle from a sudden stop.
2. Background Art
Conventionally, an HST comprising a hydraulic pump and a hydraulic motor, which are fluidly connected through a closed fluid circuit, has a control arm which engages with a movable swash plate of the hydraulic pump. The quantity of oil discharged from the hydraulic pump can be changed in a stepless fashion by rotation of the control arm. The control arm is regularly biased by a neutral return spring, whereby the movable swash plate is automatically returned to its neutral position by the neutral return spring when an operator stops speed changing operation and releases the operational power. Accordingly, the movable swash plate suddenly returns to the neutral position when the operating force is released at a high speed position of the movable swash plate and operating oil which has smoothly circulated in the closed fluid circuit theretofore is blocked in the hydraulic pump so that the rotation of the hydraulic motor is suddenly stopped. In other words, a dynamic brake is applied. Therefore, a control arm is often provided with a shock absorber to prevent sudden returning to its neutral position, as disclosed in U.S. Pat. No. 5,094,077, for example.
A shock absorber which is provided to prevent the control arm from suddenly returning to its neutral position is disposed on the outside of the HST housing so as to enable mounting and exchange thereof. However, a shock absorber which uses gas or fluid for shock absorption is heated by radiation of the housing which is heated by the rising temperature of the oil therein and is affected by the temperature of the outside air. The gas or fluid expands or contracts because of such a change of temperature so that operation of the shock absorber differs according to temperature or, in some cases, the neutral position can not be fixed at a high or a low temperature.
Further, a shock absorber mounted on the outside of the housing requires space for it to be mounted which can restrict the shape of the axle driving apparatus. As a result, the entire axle driving apparatus becomes larger. Also, because the shock absorber is mounted outside of the housing, the movable portion of the shock absorber is conventionally covered with a protector such as a rubber boot to avoid penetration of water, dust or other foreign matter, and therefore, requires an increase in the number of parts and in the cost.
The main object of the present invention is to provide a mechanism of returning to neutral for an axle driving apparatus, which can moderate a shock of deceleration by returning to neutral in advancing or reversing of a vehicle.
The second object of the present invention is to provide a mechanism of returning to neutral for an axle driving apparatus, which can moderate a starting shock of vehicle for advancing and reversing.
The third object of the present invention is to provided a mechanism of returning to neutral for an axle driving apparatus, which can prevent the vehicle left on a slope in neutral from suddenly self-descending.
The axle driving apparatus regarding to the present invention is constructed as follows:
Axles and a hydrostatic transmission as a combination of a variable capacity hydraulic pump and a hydraulic motor for driving axles are contained in fluid sump formed within a housing for the axle driving apparatus. The hydrostatic transmission is constructed such that suction and discharge ports of the hydraulic pump are respectively connected with discharge and suction ports of the hydraulic motor through fluid passages. The hydrostatic transmission is provided with a capacity changing member, which can be shifted between a neutral position making the hydraulic pump discharge substantially no fluid and an acting range making the hydraulic pump discharge fluid. A biasing member biases the capacity changing member in the acting range so as to return it to the neutral position. Orifices are provided which are connected to the fluid passages. A shutting member faces the orifices so as to be operable to open and shut the orifices toward the fluid sump. The shutting member is interlocked with the capacity changing member so that at least one of the orifices is open to the fluid sump during returning of the capacity changing member from the acting range to the neutral position. Pistons with the orifices provided therethrough are slidably fit to the oil passages and the outer surfaces thereof are pressed against the shutting member by hydraulic pressure in the oil passages. The shutting member is provided with grooves so as to communicate with the orifices of the piston pressed as such. Other biasing members for pushing the pistons are smaller than the biasing member for returning the capacity changing member to neutral.
In such a construction, to achieve the main object, a predetermined position is provided in the acting range, so that when returning the capacity changing member in the acting range to the neutral position, the shutting member shuts both the orifices of the pistons until it reaches the predetermined position, and after the capacity changing member passes the predetermined position until the neutral position, it allows one of the orifices to open toward the fluid sump while still shutting the other orifice.
Alternatively, the shutting member is constructed such that during returning of the capacity changing member in the acting range to the neutral position, it shuts one of the orifices and allows the other orifice to open toward the fluid sump at any position in the acting range.
To achieve the second object, the shutting member is constructed such that at the moment that the capacity changing member leaves the neutral position, it allows both the orifices to open toward the fluid sump, and when the capacity changing member is further apart from the neutral position, the shutting member shuts one of the orifices.
To achieve the third object, the shutting member is constructed such that when the capacity changing member is in the neutral position, it allows both the orifices to open partly toward the fluid sump, or it allows both the orifices to open fully in case that the orifices are diametrically small enough, or the shutting member shuts both the orifices in case that another member for giving a dead zone of the neutral position is provided, or it shuts one of the orifices.